English does not capitalize nouns unless they are so-called proper nouns, by which I do not mean proper in the British sense of being "in order" or "the way a thing should be done," but instead nouns that are used as actual names for people or places or ideas that are represented in titular form. The other variety of nouns are called common nouns, and they refer to everyday things that are not singled out for such special naming. Unlike German, in which every noun is capitalized, common nouns in English are not singled out for special treatment (unless, of course, they come at the beginning of a sentence, or are used as class identifiers in a poetic sense: occasionally you will see truth or beauty written as Truth or Beauty, signifying that these represent special philosophically representative uses of the terms).

Today can be a noun. When I say "Today is going to be a great day", I use today as a noun. Unlike the names for days of the week though, it can be used on any day to refer to "this present day".
–
TragicomicJan 31 '11 at 8:24

1

Why is "today" in "Today is going to be a great day" any more of a noun deserving capitalization than "this" in "this is going to be a great day"?
–
RedGrittyBrickJan 31 '11 at 11:35

1

About why the names of the days of the week are capitalized unlike, say, the names of seasons, I suspect the answer may lie in the etymology of these names (not sure, though).
–
TragicomicJan 31 '11 at 12:12

1

@Christian: my comment was a subtle hint that your answer seems to equate noun and name, which are not the same. The distinction you are looking for is that between common nouns and proper ones.
–
RegDwigнt♦Jan 31 '11 at 12:18

1

@Christian: thanks for the edit, but pronoun is something different yet again. Me, he, and ours are pronouns; Monday and Friday are not.
–
RegDwigнt♦Jan 31 '11 at 14:54